The invention relates to curable mixtures of glycidyl compounds and aminic hardeners, which comprise heterocyclic tertiary amines as cure accelerators and which, while having a long potlife, still have a rapid full cure rate.
Curable mixtures based on glycidyl compounds and aminic hardeners are widely used in industry for coating and improving the quality of metallic and mineral surfaces.
The amines used are especially aliphatic, cycloaliphatic, aromatic or araliphatic compounds, and also polyaminoamides, based on mono- or poly-basic acids, which may or may not contain imidazoline groups, as well as adducts thereof with epoxy resins.
Such compounds are described in Lee and Neville, Handbook of Epoxy Resins, 1967, Chapters 6/1 to 10/19.
Although the curable mixtures based on epoxy resins and such amine compounds usually have an adequately long potlife, once they have been applied to the various substrates their curing rate is for many fields of use too slow.
In addition, the viscosity of many compounds, especially the viscosity of the higher epoxy resin adducts and of the polyaminoamides or adducts thereof, is relatively high, especially in the low temperature range.
The curing rate can be increased by the addition of suitable cure accelerators.
For that purpose, in some systems Mannich bases of tertiary amines, such as, for example tris(dimethylaminomethyl)phenol (DMP30), are used.
A disadvantage of such compounds, however, is that they cannot be used for a large number of applications because of their strong tendency to yellow and their relatively high viscosity and also, especially, their substantially curtailed potlife.
For that reason, in some applications phenol-free accelerators that contain at least one tertiary amine group are employed. A typical representative of that class is dimethylaminopropylamine. Although some of the above-mentioned problems, such as, for example, the tendency to yellow, are not so pronounced, such accelerators likewise have the disadvantage that, while the curing rate is appreciably increased, the pottife is substantially shortened.
The problem underlying the present invention is therefore to make available curable mixtures, based on epoxy resins and amines, that have a comparatively low viscosity and in which the accelerators do not have a tendency to yellow, that are toxicologically harmless and, while having comparably long or prolonged potlives, have an appreciably faster curing rate than the non-accelerated mixtures, and of which the physical, mechanical and optical properties remain at a high level.
This problem is solved by curable mixtures based on epoxy resins and conventional aminic hardeners that comprise as cure accelerators heterocyclic amines which can be prepared by the reaction of formaldehyde with amines, each of which contains a primary and a tertiary amine group.
The invention accordingly relates to curable mixtures based on epoxy resins and aminic hardeners, where appropriate with the concomitant use of solvents, plasticisers, UV stabilisers, dyes, pigments, fillers, wherein there is used as cure accelerator from 1 to 20% by weight, based on epoxy resin, of at least one heterocyclic compound of the general formula (I) 
wherein R1, R2 and R3, which may be the same or different, are
xe2x80x94(CH2)axe2x80x94Nxe2x80x94[(CH2)bxe2x80x94CH3]2
radicals in which a=2 or especially 3 and b=1 or especially 0.
The invention relates also to curable mixtures consisting of:
a) from 30 to 70% by weight, based on epoxy resin+hardeners, of epoxy resin having an EP value of from 0.4 to 0.56
b) from 25 to 70% by weight, based on epoxy resin+hardeners, of aminic hardeners
c) from 1 to 10% by weight, based on epoxy resin, of cure accelerator of the general formula (I).
Further subjects of the invention are characterised by the claims.
The heterocyclic amines used in accordance with the invention can be prepared by the reaction of formaldehyde with amines of the general formula (II)
H2Nxe2x80x94(CH2)axe2x80x94N[(CH2)bxe2x80x94CH3]2 xe2x80x83xe2x80x83(II)
wherein a and b are as defined above, in a molar ratio of formaldehyde to amine of from 0.9:1 to, preferably, 1:1.
The preparation is generally carried out by introducing the amine into a reaction vessel and adding formaldehyde, preferably paraformaldehyde, thereto in portions. The reaction proceeds exothermically. The addition of the aldehyde is therefore controlled in such a manner that a reaction temperature of approximately 100xc2x0 C. is not exceeded. According to the invention, the reaction is carried out preferably at a temperature of approximately from 30xc2x0 to 80xc2x0 C. The water of reaction formed is removed from the reaction mixture after the whole amount of the formaldehyde has been added, its removal being effected by increasing the temperature to approximately 130xc2x0 C. and, if necessary, applying reduced pressure. The residue which remains can be used as cure accelerator without further purification.
The epoxide compounds used in accordance with the invention are customary commercial products having more than one epoxy group per molecule that are derived from mono- or/and poly-hydric, mono- or/and poly-nuclear phenols, especially bisphenols, and novolaks. A comprehensive list of such di- or poly-phenols may be found in the handbook xe2x80x9cEpoxidverbindungen und Epoxidharzexe2x80x9d by A. M. Paquin, Springer Verlag, Berlin, 1958, Chapter IV, and Lee and Neville xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, 1967, Chapter 2.
It is also possible to use mixtures of two or more of the epoxy resins. Preference is given to glycidyl compounds based on bisphenol A (4,4xe2x80x2-dihydroxy-diphenylpropane-2,2) having epoxy values of from 0.4 to 0.56.
It is also possible to use mixtures of the epoxy resins with so-called reactive diluents, such as, for example, monoglycidyl ethers of phenols or mono- or di-functional aliphatic or cycloaliphatic alcohols. Such reactive diluents are used especially to reduce viscosity and should be employed only in small amounts since they have an adverse effect on the final properties of the duromer.
The epoxy resins mentioned by way of example can be used both for the curable combinations and for the preparation of the hardener/epoxide adducts that are likewise suitable for use in the process according to the invention.
The hardeners used in accordance with the invention are aliphatic, cycloaliphatic, araliphatic or aromatic amines, aminoamides, which may or may not contain imidazoline groups, and adducts thereof with glycidyl compounds, containing more than two reactive amine hydrogen atoms per molecule. Those compounds form part of the general state of the art and are described, for example, in Lee and Neville, xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, McGraw Hill Book Company, 1967, Chapters 6 to 10.
For the purpose of coating metal or mineral substrates, preference is given to the use of cycloaliphatic amines or polyaminoamides based on mono- or poly-basic carboxylic acids and polyalkylenepolyamines, and also adducts thereof with glycidyl compounds.
Such hardeners are customary commercial products and are marketed, for example, by Witco GmbH under the trade mark EUREDUR(copyright), especially EUREDUR (ED) 43, ED 46, ED 350.
Hardeners and epoxide compounds are used in the process according to the invention preferably in approximately equivalent amounts, that is, based on active amine hydrogen atoms and reactive epoxy groups. It is, however, also possible to use hardeners or glycidyl components in excess or in less than the stoichiometric amount. The amounts of each are generally in the range of approximately from 10 to 200% by weight based on glycidyl compounds, and are governed by the desired final properties of the reaction product. Conventional catalytic hardeners for epoxy resins can be used alongside the cure accelerators, according to the invention.
Generally, the cure accelerators according to the invention and the conventional catalytic or reactive hardeners can be added in the desired ratio to the epoxide compound separately or in the form of a mixture.
The mixing ratio of glycidyl compound to the cure accelerators of formula I used in accordance with the invention is governed by the particular compound being used and the desired final properties of the cured end products.
When liquid glycidyl ethers based on bisphenol A or bisphenol F having epoxy values from approximately 0.50 to approximately 0.56 are used, the amount of accelerator employed will generally be from 1 to 20% by weight, based on glycidyl ether; preferred according to the invention is an amount of approximately from 3 to 10% by weight.
The particular advantage of the curable mixtures according to the invention is their excellent curing behaviour at temperatures of from 10 to 40xc2x0 C. and at high relative humidity values of up to approximately 95%.
Depending, on the field of use and the intended purpose, inorganic and/or organic additives may be added to the epoxy resin/hardener mixtures according to the invention, such as finely particulate sands, talc, silicic acid, clay, metals or metal compounds in the form of filings or powder, thixotropic agents, fibrous substances, such as, for example, asbestos and glass staple fibres, pigments, flame-retardant materials, solvents, dyes, plasticisers, bitumen and mineral oils.
The curable mixtures according to the invention can be used for coating absorbent or non-absorbent substrates, such as, for example, natural stones, marble, concrete, metals, glass.
Analytical Methods
Viscosity
Measured using, a Haake rotary viscometer RV 20 according to the manufacturer""s instructions.
Colour number
Measured according to DIN 53 995 using a Lovibond colour-measuring apparatus (Hazen Farbzahl, APHA).
Amine number
Measured according to DIN 16 945
Tecam value
Value for the gelling time, measured using a Tecam Gelation Timer GT3 from Techne, Cambridge, GB, at 23xc2x0 C. and 50% relative humidity. Specimen mixture of resin and hardener and accelerator=250 g.
Shore D Hardness
Measured with an apparatus of the type 38009 from Karl Frank GmbH after 1, 2, 3 and 7 days using test specimens having a diameter of 30 mm and a thickness of 6 mm.